Flexible printed circuit boards (PCBs) are known, and typically comprise a thin, flexible electrically insulating layer (such as polyester, polyimide, thermoplastic), on which is disposed a patterned electrically conducting layer (such as copper). A further protective cover layer may be disposed over the conducting layer to protect it, the cover layer having apertures to provide electrical access to the conducting layer.
Single sided PCBs, with a single conducting layer on a single side of a flexible insulating layer, may be made by: laminating together a copper and polyimide layer; depositing a resist coating over the copper layer; patterning the resist layer by a lithographic process; etching the copper layer; stripping the resist and applying the cover layer. Holes may be created in the flexible insulating layer and/or through all the layers of the flexible PCB.
Lamination of the conducting and insulating layer may be carried out prior to patterning the layers, by feeding continuous, un-patterned sheets of the conductor and insulator into a nip roller press, which provides heat and pressure to the adhesive therebetween to bond the conductor and insulator together.
Double sided flexible PCBs are known, in which a conducting layer is provided on both sides of a flexible insulating layer. In order to make double-sided flexible PCBs, continuous un-patterned sheets are first laminated together to form a conductor-insulator-conductor layer structure. The conducting layer on both sides is subsequently patterned by carrying out the patterning process outlined above on both sides of the flexible PCB. Conducting vias may be created to provide electrical connections between the two conducting layers.
Multi-layer PCBs may be formed by laminating together single sided or double sided flexible PCBs using a static press. Such multi-layer PCBs therefore comprise at least two conducting layers and at least two flexible insulating layers.
Relative to laminating un-patterned blanket layers, which may be achieved using a continuous process, laminating multiple flexible PCBs (each with patterned conducting layers) is demanding, and typically requires greater duration under heat and pressure. Lamination of multi-layer PCBs is therefore typically undertaken using a static press, and typical curing cycles using an appropriate adhesive may be in excess of an hour. The size of the static press used in such a long duration lamination presently limits the maximum size of multi-layer flexible PCBs.
Flexible PCBs may be used as replacements for wiring harnesses. In many such applications, multi-layer flexible PCBs are desirable, for instance due to their potential to provide shielding from electromagnetic interference, and/or to provide a greater density of conducting tracks by stacking. Some aerospace applications require that the total harness length is longer than can typically be accommodated by a flat press. Although several multi-layer flexible PCBs may be connected in series, the connectors form potential failure points, and may be problematic.
The present applicant has identified that nip roller presses are not suitable for laminating together the component layers of multilayer flexible PCBs. The circuit pattern results in varying layer thickness across the width (perpendicular to the direction of movement in a roller press) and along the length (parallel to the direction of movement in the roller press). As nip rollers apply a line force which is related to thickness, rather than a fixed pressure, this variation of thickness results in a variation of line force and a tendency to disturb the layers being bonded, resulting in misalignment and/or layer creasing.
WO 2008/150622 discloses a method of making a multi-layer flexible circuit of arbitrary length by using a continuous lamination process to combine a plurality of insulating layers with a plurality of conducting layers. WO 2008/150622 teaches a method of combining self supporting alternating layers of insulating and conducting material by extending them from a roll, and laminating them together, wherein through holes in the layers are used to maintain alignment therebetween. WO 2008/150622 is silent on the lamination process by which the layers are combined, and does not disclose a process which is suitable for combining multiple patterned layers. Furthermore, the method of WO 2008/150622 is not suitable for use with layers that comprise multiple separate (i.e. disconnected) regions, since each layer is individually extended from a roll before being combined together. A layer comprising multiple separate regions would therefore fall apart prior to lamination following the teaching of WO 2008/150622.
US 2001/0018796 discloses a method of making a multilayer circuit structure in which two circuit substrates are combined by passing them through a nip between a roller element and a body while heat and pressure are applied. The circuit substrates are provided with tooling holes through which tooling pins on the roller element engage, thereby maintaining alignment between the substrates as they are combined. As described above, the present applicant has identified that this type of hot roll lamination process is not suitable for laminating PCBs with topology resulting from patterned conducting layers.
A method of manufacturing multi-layer flexible PCBs of arbitrary length which can accommodate topology arising from discontinuous, irregular and/or patterned conducting layers is therefore desirable.